1. Field of the Invention
This invention relates to the field of communications, and in particular to CDMA decoding techniques in a multi-channel handset in IS2000.
2. Description of Related Art
Code Division Multiple Access (CDMA) is a commonly used spread-spectrum communications technique wherein an information signal is encoded by one of many code sequences before it is transmitted. The received signal is decoded by the same code sequence to reproduce the original information signal. Transmissions from multiple transmitters can be simultaneously communicated via a common channel by employing different code sequences for each transmitter, provided that the code sequences have particular uniqueness characteristics. The uniqueness characteristics of acceptable codes substantially provide that a coherent output will only be produced when the received signal corresponds to a signal that is encoded using the same code sequence. Each code is orthogonal, or non-correlated, to each other, so that signals that are not encoded using the same code sequence as the decoding code sequence are decoded as noise signals. In a conventional CDMA system, such as a cellular telephone network, the network controller, or base station, allocates and deallocates code sequences on demand, so that each of the transmitters can transmit over the same network without interference from other transmitters.
Conventional CDMA receivers include multiple correlators, each using the same code sequence but at different phase delays. Ideally, only the correlator that is synchronous with the transmitted code would produce a coherent output, and each of the out of phase correlators would decode the received signals as noise. Typically, however, the receiver receives multiple copies of the transmitted signal, at different phases, caused by reflections of the transmitted signal off building and such. The phase differences are caused by the different paths that the reflected signals take from the transmitter to the receiver. This multipath effect is used advantageously by conventional CDMA receivers by combining the outputs of the multiple correlators that receive a reflection of the same transmission, at a different phase.
The use of cellular communications has increased dramatically, and is expected to continue to increase. This increase is associated with an increase in the number of users of cellular communications, as well as an increase in the amount of communications required per user.
With this increased use of cellular communications, the requirement for different codes has increased. The commonly used IS-95 standard, circa 1995, requires the use of 64 bit orthogonal Walsh codes, whereas the newer IS2000 standard, circa 2000, allows the use of both Walsh codes and Quasi-Orthogonal Function (QOF) codes of different lengths. The use of codes of different lengths, and codes that are not necessarily completely orthogonal with every other code has led to an increase in cross-channel interference. In addition to the interference caused by the use of non-orthogonal codes, interference may also occur between codes of different length, because a short code may, for example, be non-orthogonal to particular segments of a longer code.
The increased requirement for communications per user includes the requirement for multiple channels per user handset. For example, handsets are available that provide for the simultaneous communications over a voice channel and a data channel. A conventional multi-channel handset includes multiple correlators, each correlator being operated with a code associated with one of the channels. For example, an example two-channel receiver may contain six correlators, three operating with one code, at different phases from each other (for multipath reception) and three operating with the other code, also at different phases from each other.
The use of non-orthogonal codes also has an adverse effect on receivers that use multiple correlators to overcome, or take advantage of, multipath effects. If a correlator detects a slight correlation, this correlation may be the result of a reflection of the desired signal, and therefore combining its result with the results from other correlators will be beneficial. If, however, the correlation is the result of an other, non-orthogonal, channel's transmission, combining its result with the results from other correlators will be detrimental.
A variety of techniques for overcoming the interference caused by the increased use of non-orthogonal codes are available for use at a base station. U.S. Pat. No. 5,506,861 “SYSTEM AND METHOD FOR JOINT DEMODULATION OF CDMA SIGNALS”, issued 9 Apr. 1996 to Bottomley et al, and incorporated by reference herein, presents a scheme for demodulating single and multiple signals in a multipath, time-dispersion environment, with or without taking intersymbol interference into account. This scheme determines each bit, or symbol, value on each of a plurality of channels by minimizing an error function that is based on the totality of channels. The minimization process also includes a dynamic adjustment of filter coefficients within each channel. The complexity of this scheme, however, renders it infeasible for embodiment in a handset, and this scheme does not directly address codes of varying sizes.